The present invention relates to a process for mixing a liquid or mixture of a liquid and a fine solid present in an essentially self-contained vessel, with the proviso that the liquid or mixture fills only part of the internal volume of the vessel occupiable by a fluid phase, and the remaining occupiable internal volume of the vessel is filled by a gas phase, comprising supply of essentially the same liquid or essentially the same mixture into the vessel as a motive jet of a jet nozzle which is disposed in the liquid or in the mixture in the vessel and has a motive nozzle and a momentum exchange chamber into which the outlet of the motive nozzle points.
The storage of liquids or of mixtures of a liquid and a fine solid in essentially self-contained vessels is common knowledge (for example for the purpose of storage). Typically, such vessels are also referred to as tanks. Normally, the vessels are not entirely self-contained, but rather generally have, for example, at least one withdrawal point through which it is possible to withdraw from the contents stored in the vessel as required, for example by means of a pump. Correspondingly, the vessel typically also has at least one feed point through which the contents to be stored can be supplied to the vessel. Shutoff members (for example valves or ballcocks) normally enable the liquid or the mixture to be let in and out, and simultaneously ensure that leaks are prevented when the vessel is inactive. In a similar manner, instruments for measuring temperature, fill level and pressure in the tank (vessel) can be introduced into the vessel.
Normally, the liquid or mixture of a liquid and a fine solid which is to be stored in the tank does not completely fill the internal volume occupiable by a fluid (gaseous or liquid) phase. Instead, for a wide variety of different reasons, some of this internal volume is occupied by a gas phase. When the liquid or mixture is stored at atmospheric pressure, the essentially self-contained vessel can in principle be open to the atmosphere on the gas phase side (for example via an offgas system leading through a flare (or another offgas purification system (for example gas scrubbing))). The opening cross section is normally such that it is firstly sufficiently small and secondly such that the gases balance in the course of filling and emptying of the vessel with significant pressure drop. Typically, the mean diameters of such opening cross sections are ≦25 cm (at fill volumes of typically≧100 m3, frequently up to 10 000 m3). Alternatively, devices for pressure release in the event of impermissible elevated pressure or reduced pressure, which seal tight to the response pressure, which may be at or above or below atmospheric pressure, are typically likewise installed into the relevant storage vessels (for example non-return valves). Frequently, the fill level in the storage tank is determined continuously at predefined heights in the gas and in the liquid phase by metering in a small amount of a measurement gas (based on the volume of the gas phase in the vessel, generally ≦1% by volume/h). When the contents are known, the fill level is calculated directly from the difference of the metering pressure required for this purpose in each case.
In many cases, it is necessary that the contents of such a storage tank which are variable over time as a result of withdrawal and/or addition are mixed from time to time or constantly in order to increase or to ensure its homogeneity. The causes of this may be a wide variety of reasons. When the contents of the vessel are a mixture of a liquid and a fine solid (for example a slurry), there is frequently the risk that the fine solid settles out under the action of gravity during the storage in the tank, and the tank contents thus demix in the course of time. In the case of a withdrawal from the storage tank, it would then possibly, for example, no longer be the desired mixture which is withdrawn but rather only the liquid present therein. Examples of the aforementioned case include aqueous polymer suspensions. Depending on the specific weight of the liquid phase the fine solid present therein in disperse distribution can also cream and become enriched in the liquid/gaseous phase interface. One possible example of this is polymer dispersions (also aqueous polymer dispersions).
When only a liquid is stored in the tank (vessel), this may likewise be multiphasic (for example an emulsion; examples include oil-in-water emulsions and water-in-oil emulsions) and demix in the course of prolonged storage without intermediate homogenization, which is normally undesired.
However, a chemically homogeneous liquid too may form undesired physical inhomogeneities in the course of storage. These may consist, for example, of an inhomogeneous temperature distribution (for example caused by solar irradiation on one side of the tank). The consequence of this may, for example, be undesired crystal formation or unwanted decomposition of the liquid stored. Frequently, for the purpose of maintaining a desired storage temperature, a portion of the stored liquid may also be withdrawn continuously, conducted through a preferably indirect heat exchanger and then recycled into the storage tank. In this case, the storage vessel operator typically aims for very rapid temperature balancing between liquids still present in the storage vessel and liquid recycled into it via the heat exchanger by suitable rapid mixing.
For the safe storage of free-radically polymerizable compounds (or solutions comprising them), for example acrolein, methacrolein, acrylic acid, methacrylic acid and/or esters thereof (especially the C1- to C8-alkyl esters), not only is careful temperature control of the liquid tank contents required. Instead, so-called inhibitors (free-radical scavengers) have to be added to the aforementioned, generally at least monoethylenically unsaturated, organic compounds (monomers), in order to avert and to prevent the occurrence of an accidentally initiated, undesired free-radical polymerization. In many cases, such inhibitors display their full effect only in the presence of molecular oxygen (which may in turn itself be an inhibitor). For this reason, such monomers are normally stored under a gas atmosphere comprising molecular oxygen (cf., for example, WO 2005/049543 and U.S. Pat. No. 6,910,511), and it should be ensured that the liquid monomer (or its solution) does not become depleted of the molecular oxygen dissolved therein. The latter can occur, for example, when the monomer temporarily crystallizes out locally and then goes back into solution. The resulting local depletion of molecular oxygen can equally be counteracted by appropriate mixing.
Should undesired free-radical polymerization of the tank contents be triggered in spite of the above-described precautionary measures, it can be counteracted by adding a medium for immediately ending the free-radical polymerization to the tank contents within a very short time and distributing it over the tank contents very rapidly (cf., for example, WO 00/64947, WO 99/21893, WO 99/24161, WO 99/59717). In this case too, very uniform and rapid mixing of the tank contents is required after the medium has been added.
As shown, for example, in FIG. 1, the liquid contents 20 of a tank 10 can be mixed by bubbling or jetting through a shower head 30 a suitable gas into the tank close to the bottom. The gas bubbles 40 ascending within the liquid tank contents 20 accomplish the desired mixing by entraining liquid. The entire (in principle, the mixing action even increases from the bottom upward) liquid vessel contents is thus covered and mixed efficiently by such a large-volume flow irrespective of the height of the liquid level. However, a disadvantage of such a procedure is the constant demand for a suitable mixing gas during the mixing (on the industrial scale, comparatively large gas volume streams are required to mix the tank contents). Moreover, this gas has to be conducted back out of the tank constantly. In the case of bubbling through the liquid tank contents to be mixed, it additionally normally becomes saturated with the liquid present in the tank and, owing to this loading (for example in the case of a stored organic liquid), it frequently cannot be released into the environment in a simple manner. Instead, in most cases, comparatively complicated (expensive) offgas treatment (for example combustion (in these cases, the gas which necessarily escapes as the tank is filled is combusted in a flare) or washing) is required. In principle, the mixed gas conducted out of the tank can also be recycled back into it for bubbling through the liquid contents thereof. However, it disadvantageously necessarily requires a separate cycle gas compressor which recompresses the offgas to the pressure at the vessel bottom. Such compressors are not only expensive but also cause a high level of maintenance and a not inconsiderable energy demand.
Alternatively, the tank contents can be mixed by means of a stirrer. However, this requires a separate drive source and a drive shaft conducted through the vessel wall. However, the sealing of rotating elements conducted through a vessel wall is generally found to be particularly difficult. Moreover, in the case of large fill volumes of a tank (industrial scale fill volumes for storage tanks are typically from 100 m3 to 10 000 m3, frequently from 200 to 1 000 m3 or from 300 to 800 m3, characteristically 500 m3), the manufacture of a stirrer is already comparatively expensive.
Against this background, as shown, for example, in FIG. 2, it has found to be appropriate to mix the liquid tank contents 20 by withdrawing therefrom, with the pump available for tank withdrawal, a portion of the liquid or mixture of a liquid and a fine solid 20 stored in the tank (vessel) 10, and recycling at least some of the portion withdrawn through a motive nozzle 50 which is disposed close to the bottom of the tank and is directed upward (in the simplest case a flow channel with cross section narrowing in flow direction, in which the pressure energy of a liquid flowing through is converted with low losses to additional kinetic energy, and the liquid stream is thus accelerated) as a (motive liquid) liquid jet (motive jet) into the tank.
In the course of this, the liquid jet directed upward, according to the laws of the free jet, along its path through the liquid present in the tank, is sucked in by the liquid, and the liquid media become mixed.
Alternatively or additionally, for the purpose of mixing, the filling (refilling but also first filling) of the vessel with the liquid or mixture can be effected in such a way that the liquid or mixture is supplied via an aforementioned motive jet.
However, a disadvantage of this method of mixing is that the mixing action of the free jet only captures a comparatively restricted space around it, so that the mixing action achieved is normally not entirely satisfactory (FIG. 2).
A further disadvantage is that the liquid jet (especially in the case of falling fill level in the tank), owing to its comparatively high mea momentum density (and speed), leaves the liquid phase present in the tank comparatively easily (breaks through the phase interface between liquid and gaseous phase), and this leaving may be accompanied by intense droplet formation (spray formation) within the gas phase. This is disadvantageous especially when the tank contents comprise an organic liquid (for example acrolein, methacrolein, acrylic acid, methacrylic acid, the esters of these acids or other organic monomers) whose gas phase may be explosive in the presence of molecular oxygen (cf., for example, DE-A 10 2004 034 515). Firstly, the finely distributed droplets in the gas phase increase their content of organic material, as a result of which a gas phase which may not have been explosive beforehand becomes an explosive gas phase, and the droplets formed regularly experience, in their flight through the gas phase, as a consequence of friction, electrical charging of their surface. Spark discharge which accrues as a consequence is capable of triggering ignition. When the droplets are those of an aqueous polymer dispersion, these may also, for example, film irreversibly in an undesired manner on their path through the gas phase and disrupt the polymer dispersion in later uses.
When the tank contents are the slurry of a fine solid in a liquid, the solid thrown onto the inner wall of the vessel by the jet which breaks through the phase interface may be capable of adhering to it, which removes it from the slurries stored in the vessel.
However, spray formation which is established as described above is also disadvantageous in the case of another liquid in that, inter alia, the small spray droplets have an elevated vapor pressure. This causes undesired evaporative cooling, which impairs the temperature constancy of the tank contents.
In order to intensify the mixing (cf. Chemie-Ing. Techn. 42, 1970, p. 474 to 479), as shown for example in FIG. 3, a mixing chamber 2 (open at the inlet and outlet) is arranged beyond the motive nozzle 1. As a result, the liquid present in the tank space is not, as in the case of a free jet, sucked in along the jet path, but rather the amount conveyed according to the law of momentum has to enter the inlet cross section of the mixing chamber 2 (also referred to hereinafter in simplified terms as a momentum exchange chamber or as a momentum exchange tube; cross section need not, though, necessarily be circular; however, the tubular embodiment is appropriate from an application point of view) through an inlet or suction orifice 3. This arrangement of motive nozzle and mixing chamber (which is, for example, connected downstream of the motive nozzle as a short tube with larger cross section) will be referred to hereinafter as a jet nozzle. In it, the motive jet with comparatively high speed enters a momentum exchange chamber which is comparatively small in comparison to the tank volume (frequently, the volume of the momentum exchange chamber is only from approx. 0.0001 to 1% of the internal volume of the tank) and sucks in a circulating amount of the liquid present in the tank as it does so. A manufacturer of such suitable jet nozzles is, for example, GEA Wiegand GmbH in D-76275 Ettlingen.
As shown, for example, in FIG. 4, the mixture which flows out of the momentum exchange tube 2 has an already significantly weakened momentum of its elements (a reduced mea momentum density) in comparison to the motive jet generated by the pump 60, which lowers the above-described probability of exit with droplet formation (spray formation) (it will enter only at a comparatively lower level of the phase interface and with weakened mean exit momentum density. As shown, for example, in FIG. 5, together with the suction acting from below, the outflow directed upward out of the momentum exchange tube 2 forms large-volume circular flow fields with continuous field lines, which, in the case of a jet nozzle directed obliquely upward and preferably mounted in the tank so as to be slightly raised (cf., for example, Acrylate Esters, A Summary Of Safety And Handling, 3rd Edition, 2002, compiled by Atofina, BASF, Celanese, Dow and Rohm & Haas), causes improved (especially more complete) mixing compared to the motive nozzle 1, which, however, still has room for improvement. Furthermore, as shown, for example, in FIG. 6, when the fill level (the phase interface) falls below the suction level, the motive jet here too passes unhindered through the momentum exchange tube and sprays to form fine droplets with the risks already described (FIG. 6). In general, the motive jet liquid, before it enters the jet nozzle, therefore has to flow through valves which, when the fill level in the tank goes below a predefined level, close and prevent flow through them.
In view of this prior art, it was an object of the invention to provide an improved process for mixing liquid tank contents, which can be applied to all above-described problem cases and not least also enables more rapid mixing.